Why? The most important case would be a file association or GUI
button I would guess, and there waiting or not waiting does not tend
to make a difference, does it?

I got complaints before about processes hanging around when gnuclient
was waiting on w32. (It was actually quite comical since each gnuclient
had its own command window (console window) on w32. I did not notice
since I always had quite a lot of windows.)

That was actually why I started looking into the problem with waiting
and non-waiting clients and automatical start of Emacs ;-)